This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-395288, filed Dec. 26, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an optical disk device using a PRML method, in particular, to a PRML method optical disk device that performs a maximum decoded offset process.
2. Description of the Related Art
Recently, there has been widely spread an optical disk device that performs a recording and reproducing process to an optical disk such as a DVD (Digital Versatile Disk), which has been developed variously and produced. For example, as a method of recording and reproducing process of an optical disk device, there is a PRML (Partial Response and Maximum Likelihood) method.
In a conventional optical disk device that does not have characteristics of the present invention, in general, information recorded in an optical disk is reproduced as faint analog signal with use of a PUH (Pick Up Head). The analog signal is amplified by a preamplifier to the sufficient signal level, and then an offset of the reproducing signal is adjusted by an offset controller. After the offset adjustment, the reproducing signal is transmitted to an ADC (Analog to Digital Converter) circuit and a PLL (Phase Lock Loop). In the PLL, a clock is generated from the timing when the reproducing signal crosses the zero level, and the generated clock is transmitted the ADC circuit. In the ADC circuit, the reproducing signal is sampled with the clock from the PLL, and transferred into a digital signal. From the digitized reproducing signal, binary decode data are obtained by a viterbi decoder.
In such an optical disk device, in a case where a great deal of asymmetry that is asymmetric element is included in the reproducing signal, if an offset value is determined so as to achieve sufficient performance of the PLL circuit and added, the viterbi decoder cannot be performed sufficiently. On the contrary, when the offset value is set so as to achieve the performance of the viterbi decoder and added, the PLL circuit cannot be performed satisfactorily.
Further, a problem lies in that, when it is attempted that the asymmetry (asymmetric element) is eliminated by obtaining the average level, in all of the reference levels, for example, about the level-7 in PR (1, 2, 2, 1) characteristics to use a reference level of the viterbi decoder, the circuits become very complicated.
The present invention has been developed in consideration of the aforementioned circumstances and an object thereof is to provide an optical disk device through which high identification performance can be obtained by adding simple offset controller even in a case where a reproducing signal includes a great deal of asymmetry (asymmetric element).
The invention is an optical disk device characterized by comprising: reproduction signal detecting section which rotates an optical disk at a predetermined number of revolutions, irradiates a laser beam, and detects a reproduction signal corresponding to a waveform pattern of the reflected wave; a first offset section which performs an offset with a first offset value to the reproduction signal detected by the reproduction signal detecting section; A/D converting section which A/D converts the reproduction signal and outputting a digital reproduction signal based on the reproduction signal offset by the first offset section; second offset section which performs an offset with a second offset value different from the first offset to the digital reproduction signal from the A/D converting section; and maximum likelihood decoding section which decodes the digital reproduction signal offset by the second offset section with use of a maximum likelihood decoder so as to output binary decoded data according to the predetermined information.
That is, according to the present invention, while an offset is performed by obtaining an offset value for PLL circuit, which is reference of an A/D converter of the detected reproduction signal, another offset value for a maximum likelihood decoder, e.g., viterbi decoder is independently obtained and a reproduction signal is offset according to the later obtained offset value. Therefore, it is possible to eliminate asymmetry (asymmetric element) stably with the offset value for the viterbi decoder and decode reproduction information with high identification accuracy, while unfailingly A/D converting the reproduction signal.
In addition, the invention is an optical disk device characterized by comprising: reproduction signal detecting section which rotates an optical disk at a predetermined number of revolutions, irradiating a laser beam, and detects a reproduction signal corresponding to a waveform pattern of the reflected wave; A/D converting section which A/D converts the reproduction signal and outputs digital reproduction signal; offset section which offsets the digital reproduction signal outputted from the A/D converting section with a predetermined offset value; and maximum likelihood decoding section which decodes the digital reproduction signal offset by the offset section with use of a maximum likelihood decoder so as to output binary decoded data according to the predetermined information.
That is, according to the invention, without an offset for the A/D converter, but if an offset value is separately obtained for viterbi decoder and the reproduction signal is offset according to the obtained value, it is possible to provide an optical disk device capable of eliminating the asymmetry (asymmetric element) unfailingly with the offset value for the viterbi decoder and decoding the reproduction information with high identification accuracy. As a result, it is possible to reproduce highly accurate recording information from which the asymmetry (asymmetric element) has been eliminated through a simple circuit structure.